A control valve is generally used for a continuous control of a liquid or gas flow in different pipelines and processes. In a processing industry, such as pulp and paper, oil refining, petrochemical and chemical industries, different kinds of control valves installed in a plant's pipe system control material flows in the process. A material flow may contain any fluid material, such as fluids, liquors, liquids, gases and steam. The control valve is usually connected with an actuator, which moves the closing element of the valve to a desired open position between fully open and fully closed positions. The actuator may be a cylinder-piston, for example. The actuator, for its part, is usually controlled by a valve positioner or a valve guide, which controls the position of the closing element of the control valve and thus the material flow in the process according to a control signal from the controller.
Processes are controlled by control loops/circuits. A control loop or circuit consists, for instance, of a process to be controlled, a control valve, a measuring sensor and transmitter, and a controller. The controller gives the control valve a control signal as an analog current signal or a digital control message, for example. The measuring sensor measures a controlled variable, and the measurement product obtained is fed back to the controller, where it is compared with a given reference value. On the basis of the difference variable, the controller calculates the control for the control valve. Usually the controller functions in such a manner that it minimizes the difference variable by a suitable control algorithm, such as a PI or PID algorithm. This control algorithm is typically tuned for each valve during mounting or operation.
A known flow control implemented with a control valve comprises an entity consisting of a control valve, a controller and a flow indicator. A PID or PE type of controller receives a flow set point Qsp and a measured flow Qm. In the PID control algorithm it is assumed that the control valve is linear, i.e. that the flow is linearly dependent on the control signal, which is also referred to as a linear installed characteristic. When the control valve is linear, the controller parameters P, I and D may be fixed values. Flow gain dQ/dα (Q=flow, α=valve opening) of the installed control valve must in this case be constant in order for the control valve to operate in a stable manner and with as small an error as possible in the entire flow area to be controlled.
In practice, the characteristic of the control valve installed in the process pipeline is non-linear due to the natural characteristic of the valve, overdimensioning of the valve, pipeline losses and the pump curve. In other words, with different valve openings the flow gain varies considerably in process conditions. In this case, the PID controller gain should always be changed to adapt to each operating point in order to achieve a good flow control. Another problem is that the slope of the installed characteristic does not remain constant at the same operating point of the control valve but changes, for example, when the other control elements connected to the pipeline are performing control operations. For stability reasons, the controller parameters must often be set according to the highest flow gain of the value and the process conditions, and when changes take place, in the region of a lower flow gain the flow to be controlled has error a long time because of the slowness of the control. To alleviate this problem, the flow gain that changes regularly according to the flow may be compensated for by a tabulated gain of the controller. In practice, it is difficult to find tabular values, and they cannot be used for compensating for randomly changing process conditions.
Finnish Patent FI 53047 discloses utilization of a Cv (flow co-efficient) curve of a valve for controlling the control valve. In this known solution, the valve opening and the pressure drop Δp across the valve are measured. These are read to a control unit, in which there is stored the Cv curve of the valve as a function of the position of the closure member. By using a known flow equationQ=N*Cv*√{square root over (Δp/G)}  (1)
the control unit solves the required Cv value and, by utilizing the Cv curve, the required position of the closure member, when Q=Qsp, G=relative density of material flow and N=constant coefficient. Inaccuracies in the Cv curve of the valve, the measurement of the valve opening and the measured pressure drop across the valve appear as a permanent error in the controlled flow. The equations of the patent FI 53047 use the coefficient kv, whose value may be kv=0.857×Cv, for instance. The difference between the coefficient kv and the parameter Cv used herein is the unit used.